Disable some GNOME Overview search providers

When I open the GNOME Overview, a number of services start because they implement enabled search providers:

dbus-daemon[4651]: [session uid=1000 pid=4651] Activating service name='org.gnome.ControlCenter.SearchProvider' requested by ':1.43' (uid=1000 pid=7580 comm="/usr/bin/gnome-shell ")
dbus-daemon[4651]: [session uid=1000 pid=4651] Activating service name='org.gnome.Nautilus' requested by ':1.43' (uid=1000 pid=7580 comm="/usr/bin/gnome-shell ")
dbus-daemon[4651]: [session uid=1000 pid=4651] Activating service name='org.gnome.Calculator.SearchProvider' requested by ':1.43' (uid=1000 pid=7580 comm="/usr/bin/gnome-shell ")
dbus-daemon[4651]: [session uid=1000 pid=4651] Activating via systemd: service name='org.gnome.Terminal' unit='gnome-terminal-server.service' requested by ':1.43' (uid=1000 pid=7580 comm="/usr/bin/gnome-shell ")
systemd[4627]: Created slice app-org.gnome.Terminal.slice.
systemd[4627]: Starting GNOME Terminal Server...
dbus-daemon[4651]: [session uid=1000 pid=4651] Successfully activated service 'org.gnome.ControlCenter.SearchProvider'
dbus-daemon[4651]: [session uid=1000 pid=4651] Successfully activated service 'org.gnome.Nautilus'
dbus-daemon[4651]: [session uid=1000 pid=4651] Successfully activated service 'org.gnome.Calculator.SearchProvider'
dbus-daemon[4651]: [session uid=1000 pid=4651] Successfully activated service 'org.gnome.Terminal'
systemd[4627]: Started GNOME Terminal Server.
nautilus[8847]: Initializing

This makes the Overview slower, especially the first time one uses it, and eats more RAM.

Additionally this may be the reason why we need these workarounds in the test suite:

    # Trigger startup of search providers
    @screen.type(app_name[0])
    # Give search providers some time to start (#13469#note-5) otherwise
    # our search sometimes returns no results at all.
    sleep 2
    # Type the rest of the search query
    @screen.type(app_name[1..-1])
    sleep 4

I think we should consider disabling the least useful (for our personas) of these search providers. That's configured in the Search section of GNOME Settings.

Currently we have enabled:

• Files → not very useful without filesystem indexing, which I don't think we can rely on in a live system
• Calculator → occasionally useful, but is it worth its cost?
• Terminal → useless for most users

⇒ I think we should disable those 3.